Instability

Instability of the distal radioulnar joint may be very difficult to detect by routine radiography, as slight variations in wrist position alter the relationship of the radius and ulna. CT scanning and MR imaging, which provide cross-sectional data, may be helpful in diagnosis, but frequently the wrist must be evaluated in the neutral position with pronation and supination of the forearm, and comparison of the injured and opposite wrists is required.

Carpal instability may be lateral (which usually occurs between the scaphoid and the lunate bones); medial (which occurs between the triquetrum and the lunate or between the triquetrum and the hamate bones); or proximal (which occurs when the abnormal carpal alignment results from an injury of the radius or from massive radiocarpal disruption). Various patterns of instability occur as a consequence of ligamentous disruption. Between the scaphoid and lunate or between the lunate and triquetrum, the patterns include scapholunate dissociation, which produces dorsal intercalary segmental carpal instability (DISI), and lunotriquetral dissociation, which leads to volar intercalary segmental carpal instability (VISI). Routine radiography, arthrography and MR imaging are useful imaging techniques for these abnormalities.

Although congenital instability may occur, the leading cause of subluxations and dislocations of the glenohumeral joint is direct or indirect trauma. No single or essential lesion appears to form the basis of anterior or posterior instability of the joint. Standard arthrography and both conventional arthrotomography and computed arthrotomography have been used in the assessment of glenohumeral joint instability. MR imaging allows detection of capsular and ligamentous abnormalities that may be combined with labral alterations in patients with glenohumeral joint instability Various bone injuries may accompany glenohumeral joint instability and may be detected with MR imaging. These include fractures of the anterior or posterior portion of the glenoid rim, Hill  Sachs and trough lesions, and avulsion fractures of the lesser or greater tuberosity of the humerus.

Patellofemoral instability has symptoms and signs that may simulate those of other disorders of the knee. Routine radiographs can determine the configuration of the trochlea, the patellar shape and the relationship of the patella to the femur. Transaxial CT scans and MR images allow assessment of patellar position.

Alignment abnormalities of the spine (segmental instability) can be assessed using lateral radiographs obtained in the neutral position and during spinal flexion and extension. Radiographic findings suggestive of instability include the presence of gas within the intervertebral disc, osteophytes on adjacent vertebral bodies below the rims of the endplate (traction osteophytes), and a radial fissure in the intervertebral disc during discography. Forward or backward displacement of one vertebra on another, an abrupt change in the length of the pedicles, narrowing of the intervertebral foramina and loss of height of an intervertebral disc are other findings.

Structurally the vertebral column is composed of three columns that help determine the presence or absence of instability. The anterior column consists of the anterior longitudinal ligament, anterior portion of the anulus fibrosus and anterior half of the vertebral body; the middle column consists of the posterior longitudinal ligament, posterior portion of the anulus fibrosus and posterior half of the vertebral body; the posterior column is formed by the posterior portion of the anulus fibrosus and posterior half of the vertebral body; the posterior column is formed by the posterior osseous arch and the posterior ligaments (capsular ligaments, ligamenta flava, interspinous ligament and supraspinous ligament). The middle column is the most important in determining the potential for instability.

Three degrees of spinal instability have been defined:
· first degree (mechanical instability), in which the spine is insufficiently constrained against buckling and angulation, which places the patient at risk for progressive chronic kyphosis;
· second degree (neurologic instability), relating to injuries at risk for delayed neurologic compromise even if no deficit exists initially; and
· third degree (mechanical and neurologic instability), in which both progressive bone displacement and progressive neurologic injury may develop.

Instability of the hip may occur in newborn infants with developmental dysplasia of the hip (DDH), which may resolve spontaneously. The Ortolani manoeuvre and Barlow manoeuvre help identify children with dislocatable hips.

DR

DR